The present invention relates to a shock absorber by which the damping power thereof is controlled by the electric current for energizing the linear solenoid.
In the shock absorber used on vehicles, such as automobiles, motor bicycles or like, it is desirous that the damping power thereof is freely varied depending on the cruising conditions. For example, a system has been made known to the public, wherein the amount and rate of expansion or compression are detected so that the pressure for actuating a main valve, which is provided in the piston for opening and closing the oil passage, is freely varied by controlling energization of the linear solenoid continuously or discontinuously during each stroke of the piston (for example, by Unexamined Japanese Patent Publication No. 261528/1989 (corresponding to U.S. Pat. No. 5,090,525 and European Patent Publication No. 0330634A)).
The shock absorber used therein has the construction as shown in the schematic principle in FIG. 1. The shock absorber comprises a piston 16 for defining two main oil chambers 12, 14 in a cylinder 10, a main valve 22 provided in the piston 16 for defining a main chamber 18 and a pilot chamber 20, and an orifice 24 interposed between these main and pilot chambers 18, 20. The hydraulic pressure in the high pressure side main oil chamber 12 or 14 is transmitted to the main chamber 18. On the other hand, as the internal pressure in the pilot chamber 20 exceeds the thrusting power set by a linear solenoid 26, said main valve 22 is moved to open a main oil channel 28 (28a, 28b), which communicates with the two main oil chambers 12, 14, so as to control the damping power. Meanwhile, the in interior side of the main oil channel 28a is defined to be annular to surround the outer periphery of the main valve 22.
In this precedingly proposed system, the pressure in the high pressure side main oil chamber 12 or 14 is transmitted through the main chamber 18 and the orifice 24 to the pilot chamber 20. And, when the pressure in this pilot chamber 20 (Pilot Chamber Pressure P.sub.p) exceeds the set thrusting power P.sub.s of the linear solenoid 26 a pilot valve 30 is opened. Then, under the action of pressure drop upon passage of fluid medium through the such orifice 24 and opening of the pilot valve 30, the pilot chamber pressure P.sub.p at the downstream of the orifice 24 becomes lower than the pressure in the main chamber 18 (Main Chamber Pressure P.sub.m). Thus caused pressure difference (P.sub.m -P.sub.p) urges the main valve 22 to move upwards. Reference numeral 31 designates a spring to restore the main valve 22.
Since the pilot valve 30 is opened the pilot chamber pressure P.sub.p, as described above, the increment of the pilot chamber pressure P.sub.p is delayed owing to the interposition of the orifice 24. Such delayed response arises a problem that the response of the main valve 22 to the change in set thrusting power P.sub.s of the linear solenoid 26 is delayed.
In order to provide a quicker response in dumping the pressure in the pilot chamber, the inventor has proposed to provide the pilot valve with a valve shaft extension that passes through the main valve and is exposed to the pressure in the main chamber. This arrangement permits a more rapid opening of the pilot valve when the pressure exceeds the force of the linear solenoid. In order to improve this operation, it is necessary to provide a restrictive orifice in the conduit connecting the main chamber to the pilot chamber. Because of this orifice, there is a delay in the return motion of the main piston.
Another feature of the invention has been accomplished in view of the circumstances as aforementioned, and the object thereof is to provide a shock absorber by which the response property of the main valve responsive to change in movement of the piston or change in set thrusting power of the linear solenoid is improved.
As has been noted, a linear solenoid is employed with the constructions for holding the pilot valve in its closed position. The force necessary to cause opening of the control valve varies by varying the power applied to the solenoid and in this way the operation of the shock absorber can be very effectively controlled. However, if there is a malfunction in the electrical operation, then the operation of the shock absorber becomes uncontrolled.
It is, therefore, a still further object of the invention to provide an improved shock absorber of this type wherein the shock absorbing function can revert to a manual control in the event of an electrical failure.